1. Field of the Invention
The present invention relates to apparatuses for forming thin films on specimen substrates and apparatuses for extracting ions for forming thin films on specimen substrates, etching the surface of a thin film or improving the quality of the surface of a thin film, and more particularly a novel film forming apparatus capable of forming thin films of various materials at a high film growing rate with a high efficiency and stability for a long period of time by utilizing high-density plasma generated by electron cyclotron resonance and a novel sputtering type ion source cable of extracting various ions of high current density with a high efficiency and stability for a long period of time. Furthermore, the present invention relates to a plasma generating apparatus which is capable of generating high-density plasma in a gas under a low pressure and which can be utilized with the film forming apparatus and the ion source.
2. Description of the Prior Art
In various types of LSI production processes, the techniques regarding to the formation of thin films and to ion sources presently occupy very important positions.
In the conventional sputtering apparatuses, the discharge cannot be maintained in a stable manner in a low gas pressure range less than 10.sup.-3 Torr and plasma is generated only at a gas pressure of the order of or higher than 10.sup.-2 Torr, so that there arises the problem that a large amount of impurities are penetrated into the thin film.
The particles which contribute to the growth of a thin film are almost neutral and it has been difficult to control the energy of such neutral particles.
Meanwhile, the magnetron sputtering apparatuses (for instance, as disclosed by R. K. Waits, J. Vac. Sci. Technol., Vol. 15 (1978), pp.179-187) and the facing targets sputtering apparatuses (for instance, as disclosed by M. Matsuoka et al., J. Apply. Phys., Vol. 60 (1986), pp.2096-2102) which permit a high-rate sputtering in a gas under a low pressure have been devised and demonstrated.
In the magnetron sputtering apparatuses, the high-energy secondary electrons are trapped over the surface of the target b the effects of the magnetic field closed over the surface of the target and the electric field over the surface of the target so that high-density plasma can be generated in a gas at a low pressure. However, they have the problem that the qualities of the portions of a grown film corresponding to the eroded portions of the target and to the not eroded portions, respectively, are widely different from each other. Furthermore, when the target is made of a magnetic material such as Fe, the magnetic flux does not leak to the surface of the target so that high-density plasma cannot be generated and the kinds of thin films to be formed are limited.
FIG. 1 shows a conventional facing targets sputtering apparatus. A sputtering chamber 1 can be evacuated by a vacuum pump and gases for generating plasma are introduced from gas inlet 2. In the chamber 1, substrate holder 3 for supporting a substrate 4 is disposed. A heater 5 is incorporated in the holder 3. Two targets 6 and 7 are arranged in opposing relationship with each other. Magnets 8 and 9 for applying magnetic field on the targets 6 and 7 are incorporated in target holders 10 and 11, respectively. Target holders are electrically insulated from the chamber 1 by insulators 12 and 13. When discharge is effected in the chamber 1, the high-energy secondary electrons are confined between the targets to generate high-density plasma between targets. They have one of the special features that almost all kinds of thin films can be formed over the surface of the substrate 4 at a high deposition rate. In this apparatus, the impingement of the high-energy particles on the surface of the substrate is decreased so that this apparatus is regarded as one of the better apparatus for forming a high quality thin film at low temperatures. However, the targets 6 and 7 are disposed in opposing relationship and are spaced apart from each other by a suitable distance, so that the substrate 4 must be located at a horizontal position and the deposition rate of the sputtered particles deposited over the surface of the substrate 4 is low. Furthermore, in the case of coating a large surface of a large-sized disc or the like, there arises the problem that the deposition rate or efficiency is essentially low when the targets are disposed in the manner described above.
If it is desired to form a thin film at lower temperatures, highly active plasma with a high ionization rate must be utilized. In the case of forming reactive thin films such as oxide films, nitride films and so on, especially the reactivity of plasma is important.
As one method for forming various thin films at low temperatures there has been devised and demonstrated a sputtering type ECR microwave plasma deposition apparatus which utilizes electron cyclotron resonance (ECR) plasma and sputtering (for instance, as disclosed S. Matsuo et al. U.S. Pat. No. 4,492,620). This apparatus offers an excellent method for forming thin films at low temperatures by combining the supply of a metal by sputtering with the irradiation of a substrate with microwave plasma.
However, when the conventional ECR sputtering apparatuses are used to form thin films at a high deposition rate, the density of the microwave plasma must be increased so that there arises a defect that damages to a thin film being gown and to a substrate are increased.
Meanwhile, in order to attain the formation of a thin film at a high deposition rate, there has been devised and demonstrated an ECR sputtering apparatus in combination with the magnetron discharge on a target (for instance, as disclosed by C. Takahashi et al. J. Vac. Sic. Technol., Vol. A6 (1988), pp.2348-2352). However, according to the above-mentioned technique, one or more special magnetic circuits must be provided in order to define a closed magnetic flux distribution over a target. Furthermore, the high-density plasma exists locally on the target so that the interaction between the locally existing high density plasma and the microwave plasma is weak. As a result, there arises the problem that the efficient thin film growth cannot be carried out in the lower gas pressure range.
Meanwhile, the ion sources which utilize an ion extracting mechanism such as a grid to extract the ions produced in plasma have been widely used for forming thin films of various materials, etching the surface of a formed thin film, processing the formed thin films and so on.
As the metal ion sources, the evaporation type ion sources and the sputtering type ion sources are well known to those skilled in the art. However, the evaporation type ion sources must maintain the temperature within its furnace at high temperatures, so that the vaporized particles are ionized and consequently impurities most frequently tend to be contained in a thin film being grown. Furthermore, the extraction of ions of a material having a high melting point is difficult (for instance, as disclosed by M. A. Hasan et al., J. Vac Sci. Technol., Vol. B5 (1987), pp.1332-1339). In the cases of the sputtering type ion sources, metal ions obtained by sputtering a target in plasma are selectively extracted, but it is difficult to extract high current ions over a large area (for instance, as reported by B. Gavin, IEEE Trans. Nucl. Sci , Vol. NS-23 (1976), pp.1008-1112).
In order to realize a high-current ion source by utilizing sputtering, the plasma density must be maintained at a high level with a high efficiency. To this end, the secondary electrons emitted from the target must be efficiently confined, but the conventional ion sources cannot satisfactorily confine the secondary electrons.
An ion extracting method with a high-efficiency and a large-area is disclosed by, for instance, N. Terada et al., Proc. Int'l Ion Engineering Congress, ISIAT'83 and IPAT'83, Kyoto (1983), pp.999-1004. According to this method, a negative potential is applied to a pair of opposing targets so that the high-energy secondary electrons are confined between the targets by the magnetic field produced therebetween. As a result, high-density plasma can be generated and the extraction of metal ions can be realized with a high efficiency. However, according to this method, the ion extracting holes are formed through the target so that the target itself has a function of a grid which is means for extracting ions. As a result, it is difficult to extract ions in a stable manner for a long period of time.
It is preferable to utilize plasma generated in the low gas pressure range as much as possible in order to increase the ratio of metal ions in extracted ions. However, when the discharge on the target itself is utilized in a simple manner, there arises the problems that the stability in the low gas pressure range is not satisfactory and that the extraction of large current ions cannot be accomplished at a pressure of 10.sup.-4 Torr.
It has been therefore considered that when ECR plasma which can be generated in a stable manner even at a low pressure of the order of 10.sup.-5 Torr is combined with an ion source, the ion extraction could be realized by utilizing plasma generated in a stable manner even at the low pressure gas region.
Based upon the above-mentioned technical idea, there has been proposed an ion source in which the microwave discharge and the Pening discharge are combined (for instance, as disclosed by Y. Yoshida et al. J. Vac. Sci Techol., Vol. A6 1988, pp.2451-2456). However, even when such technique is utilized, since a loop antenna for introduction of the microwave is consumed by sputtering, not only the ion extraction in a stable manner for a long period cannot be carried out but also the technique is not essentially adapted to extract ions over a large area.
An ion shower apparatus utilizing ECR plasma was disclosed by Ono et al. (U.S. Pat. 4,450,031). However, the conventional ion sources utilizing the microwave plasma generated by utilizing ECR have no source for emitting metal ions so that it is impossible to extract metal ions. Furthermore, even when a target is disposed in an apparatus as a metal ion emission source, the density of the microwave plasma must be increased so that the ion extraction mechanism such as grids or the like receives impingement with particle in the plasma and consequently the stable ion extraction cannot be carried out.
As described above, the conventional film forming methods cannot satisfy the following conditions simultaneously:
(a) A thin film is formed at a high deposition rate without causing the damages to a thin film being grown and a substrate and an extreme temperature rise;
(b) The energy of each particle incident on the substrate is low;
(c) The ionization ratio of plasma must be maintained at a high value;
(d) The discharge in a gas under a low pressure can be carried out; and
(e) The efficiency of the deposition of atoms or ions sputtered from the target over the surface of the substrate must be high.
In like manner, the conventional sputtering type ion source technique cannot simultaneously satisfy the following conditions:
(a) a high yield (high density and large area);
(b) high plasma generation efficiency;
(c) high ion extraction efficiency;
(d) a high purity of ions;
(e) easy control of ion energy;
(f) capability of extracting ions from almost all materials including materials having a high melting point, metals and so on;
(g) exclusion of a heating and evaporating step in the ion generation process; and
(h) capability of extracting ions in a stable manner for a long period of time.
With the conventional sputtering type ion source utilizing ECR plasma, the ions of metals or materials having a high melting point can not be extracted and ion extraction can be continued for a long period of time in a stable manner.